In the process of manufacturing semiconductor integrated circuits, a wafer is subjected to various processes, such as film-formation, etching, oxidation, and diffusion. Owing to the demands of increased miniaturization and integration of semiconductor integrated circuits, the throughput and yield involving these processes need to be increased. In light of this, there is a semiconductor processing system of the so-called cluster tool type, which has a plurality of process chambers for performing the same process, or a plurality of process chambers for performing different processes, connected to a common transfer chamber. With a system of this type, various steps can be performed in series, without exposing a wafer to air. For example, Jpn. Pat. Appln. KOKAI Publication Nos. 2000-208589 and 2000-299367 disclose a semiconductor processing system of the cluster tool type.
In a semiconductor processing system of the cluster tool type, there are a plurality of transfer arms for transferring a semiconductor wafer. Each transfer arm is arranged to extend, contract, and rotate or move horizontally. The transfer arms sequentially transfer a wafer among them from a wafer cassette to processing apparatuses or vice versa.
In general, transfer of a wafer between transfer arms is not directly performed therebetween, but via a supporting mechanism disposed therebetween, which can move the wafer up and down, or a buffer table having such a supporting mechanism. The wafer is transferred onto the supporting mechanism or buffer table by one of the transfer arms, and is then transferred from the supporting mechanism or buffer table by the other of the transfer arms.
Depending on the processing manner of a wafer, the wafer is temporarily kept waiting in a transfer chamber in the middle of transfer, and another wafer is transferred by priority. In this case, the transfer chamber may be provided with a supporting mechanism or buffer table as that described above. For example, Jpn. Pat. Appln. KOKAI Publication Nos. 4-69917, 9-223727, and 2001-176947 disclose a supporting mechanism or buffer table of this kind.
In handling semiconductor wafers for a film-formation process, for example, there is a case where processed wafers are accompanied by thin films sticking to the bottom, as well as the top as a matter of course. In this case, if lifter pins or the like come into direct contact with the bottom of wafers to handle them, unprocessed wafers are contaminated with film particles transferred from the lifter pins.
Furthermore, the supporting mechanisms disclosed in the publications mentioned above include two lifters for moving a wafer up and down, which are used at the same time to transfer a wafer. Accordingly, it is difficult to use the supporting mechanisms flexibly in light of the situation.